PDH is still today the predominating multiplexing hierarchy, even though it was taken into global use already in the 1970s. A multiplexing hierarchy means that an upper hierarchy level system having a higher capacity is constructed by combining a given number of lower hierarchy level systems by means of time division multiplexing. There are three different versions of multiplexing hierarchy, one of which is used in Europe, one in the USA and one in Japan. The European system is also used in most parts of the rest of the world.
In the European multiplexing system, 31 64 kbit/s channels and one frame alignment word are multiplexed into a data stream having a rate 2048 kbit/s. This first hierarchy level signal is called E1. In the European system, the upper hierarchy level system is constructed by multiplexing four lower-level signals.
In North America and Japan, on the other hand, 24 channels and one frame synchronizing bit are multiplexed into a data stream having a rate 1544 kbit/s. This first hierarchy level signal is called T1. In the American system, the second hierarchy level system is constructed from four first level systems, the third level system from seven second level systems and the fourth level system from three third level systems. In Japan, the hierarchy is otherwise similar, but the third level system is constructed from five second level systems.
Particularly in trunk connections, however, there has been a shift to using more modern multiplexing systems that offer, for instance, better network management (e.g. easier drop and add functions for tributaries). Such newer multiplexing systems include SDH (Synchronous Digital Hierarchy) and SONET (Synchronous Optical Network). Like PDH, also SDH and SONET are based on 64 kbit/s channels in accordance with the PCM technique, and the conventional PCM signals of the PDH network can be transferred in transmission frames in accordance with the new multiplexing hierarchies.
However, an access network nowadays typically follows a n×E1 or n×T1 multiplexing hierarchy (n is an integer) on account of, for example, the smaller capacity requirement of the access network and the fact that in such a case, the same advantages are gained in the access network as with SDH in the core network (since the multiplexing between the different hierarchy levels is omitted). An additional reason for the hierarchy of an access network is that an access network uses a large number of radio connections, and thus valuable bandwidth is saved by means of the n×E1 hierarchy. (Typical values for n are 2, 4, 8 and 16.)
FIG. 1 depicts how in a conventional PDH network element incoming signals are transferred to a common transmission link TL, which can be constituted for example by a radio path, copper cable or optical fibre. The example only deals with one transmission direction (from left to right in the figure). The reverse operations are carried out in the other transmission direction. Standard PCM signals (a total of n signals) arrive at the network element from various transmission links IN1 . . . INn; in this exemplary case the signals are assumed to be E1 signals (but they can also be for example T1 signals). Each incoming signal has a dedicated input interface IFU1 . . . IFUn in an interface unit IFU, each interface performing the physical adaptation of the corresponding signal to the network element. From the interface unit, each incoming signal is connected to a frame multiplexer 11, in which a transmission frame for the next link TL is formed by multiplexing the incoming (payload) signals (n signals) and in addition a number of other signals that are denoted in the figure with a common reference HEADER DATA. Hence, a serial signal is obtained from the output of the frame multiplexer, and this signal is supplied to a transmission device 12, which is connected to link TL. Depending on the transmission medium, the transmission device still shapes the signal in different ways, but this is no longer essential to the invention.
FIG. 2 is an exemplary illustration of a frame structure that can be constructed for example by a frame multiplexer multiplexing 4 incoming 2 Mbit/s signals (E1 signals). In the example of the figure, the frame is divided into 16 sets each having 64 bits. The bits are divided into payload bits (D0–D3) and overhead bits. The payload bits are denoted in such a way that bit Di (i=0,1,2,3) belongs to the incoming E1 signal having the serial number i. The overhead bits, which are denoted by a grey zone in the figure, typically include frame alignment bits FA, justification control bits JC, additional channel bits AC, internal communication channel bits IC, and bits ED (error detection) and FS (fec syndrome) used for error detection and error correction. The bits used for rate difference equalization are not shown in the figure. Thus, a transmission frame leaving the frame multiplexer has a basic structure consisting of a payload portion (white zone in the figure), having a transmission capacity of e.g. n×E1 or n×T1, and a header portion (grey zone in the figure) in which additional information is transferred.
If it is desired to utilize a PDH network element in accordance with FIG. 1, having a plurality of 2 Mbit/s interfaces, for transfer of e.g. ATM cells, in accordance with the currently used technology this requires addition of an ATM adaptation element AE in accordance with FIG. 3, including for example an inverse multiplexer I-MUX. If the ATM cells are transported for example in a STM-1 transmission module in accordance with the SDH hierarchy, the element has, in compliance with STM-1 capacity, a standard 155 Mbit/s interface unit AIU for the incoming optical signal. In the interface unit, the incoming optical signal is converted into electrical form and the frame structure is disassembled, so that a cell stream is obtained at the output of the interface unit which is connected to a rate adaptation unit TCU. In the rate adaptation unit, the bit rate of the incoming cell stream is adapted to be correct in view of the transmission device 12 by adding or removing idle cells, i.e. cells not carrying a payload. Thereafter, the rate-adapted cell stream is connected to the inverse multiplexer I-MUX, constructing one logical link from n outgoing parallel links (OL1 . . . OLn).
Inverse multiplexing is an operation specified by the ATM Forum; by means of it a high-rate cell stream can be transferred through several parallel links. In this way, user access to an ATM network can be offered or ATM network elements can be interconnected through conventional PDH links, e.g. E1 links, which as a group offer the necessary transmission capacity. In inverse multiplexing, the cells are cyclically multiplexed onto links grouped to form one logical link whose transmission capacity corresponds approximately to the sum of the transmission capacities of the individual links belonging to the group. At the receiving end, compatible inverse demultiplexing is needed to reconstruct the original cell stream, and thus compatible devices must be added at both ends of the link or connection in order to transfer ATM cells.
In the transmission direction, the inverse multiplexer I-MUX distributes the cells arriving from the ATM layer cyclically one at a time to the links OL1 . . . OLn belonging to the group. Moreover, the transmitting multiplexer adds special cells to the cell stream of each parallel link, on the basis of which the receiving end can reconstruct the original cell stream. Cells are transmitted continuously, and thus if cells are not received continuously, the inverse multiplexer adds to the cell streams special padding cells, so that a continuous cell stream is obtained at the physical layer.
Since inverse multiplexing does not relate to the actual invention, it will not be described in detail in this context. Inverse multiplexing has been described in ATM Forum specification AF-PHY-0086.00, in which the interested reader will find a more detailed description of the subject.
From the inverse multiplexer I-MUX, the signals of all links belonging to the group are connected via output interfaces OI1 . . . OIn out from the ATM adaptation element. If the signals are E1 signals and the interfaces are in accordance with the ITU-T recommendation G.703, the signals can thereafter be directly applied to the input interfaces IFU1 . . . IFUn of the frame multiplexer 11 of the transmission device in accordance with FIG. 1. It has been presumed in the figure that the inverse multiplexer uses all input interfaces of the frame multiplexer.
However, the solution described above, utilizing inverse multiplexing/demultiplexing, has certain drawbacks. First, adding an inverse multiplexer and demultiplexer to the link or connection renders the solution expensive and complex. Furthermore, a separate ATM adaptation element will be space-consuming, as it requires its own frame in the equipment room. This is of significance particularly in newer systems in which the transmission devices are located outdoors, for example incorporated into subscriber multiplexers in street cabinets or integrated into base stations of a mobile communications system, which stations are typically located on roofs or walls of buildings.